Protection for bias filter condensers



Feb. 2, 1943. J. E. BAUDINO 2,310,054

PROTECTION FOR BIAS FILTER CONDENSERS Filed July 15, 1941 l pm wr 4 7bAGupp/g/ Peczz'fz'ad i kizar'ed Power Supply WITNESSES: INVENTOR Jbsep/zEBaz/dma Patented Feb. 2, 1943 PROTECTION FOR BIAS FILTER CONDENSERSJoseph E. Baudino, Merion, Pa., assignor to Westinghouse Electric &Manuiacturing Company,

a corporation of Pennsyl- East Pittsburgh, Pa.,

vania Application July 15, 1941, Serial No. 402,486 6 Claims. (Cl.179-171) This invention relates to protective systems and, moreparticularly, to protective circuits preventing overvoltage in certainportions of electrical transmission networks.

The invention finds particular application in transmission systems, suchas amplifiers, for preventing the damage which overvoltages may cause tocomponents endangered by surges far in excess to the normal operatingpotentials to which they are intended to be exposed.

In the operation of a high power vacuum tube as in the power amplifieror high level class B modulation amplifier of a radio transmitter, thegrid circuit is sometimes momentarily subjected to excessively highvoltage due to fiashovers from the anode. These fiashovers may be due toseveral causes, such as overmodulation or the operating characteristicsof the tubes themselves. The grid circuit is usually not designed towithstand the high voltage so produced and a breakdown of the lowvoltage large capacity condensers in the grid bias filter usuallyfollows. A breakdown of this type seriously interferes with the normaloperation of the transmitter.

A particular feature of this invention is that conditions of overvoltageto which an impedance element, such, for example, as the filtercondenser above mentioned may be subjected, is prevented automaticallyand to this end means are provided for forming a current path of highconductivity in shunt with the impedance whenever the voltage risesbeyond a certain predetermined value.

A particular advantage of this invention is that the current path abovereferred to has practically infinite impedance at the normal operatingvoltage appearing across the terminals of the impedance, whereas whenthe voltage exceeds a certain predetermined magnitude the protectivecurrent path has a very low resistance.

Another feature of this invention is that aside from the very highresistance of the shunt circuit at the normal operating voltage, thecurrent conductivity therein depends also on the polarity of theoperating voltage. In other words, conductivity depends on two factors,namely, the magnitude of the voltage and the direction of the currentflow.

In its broader aspect, the invention comprises a shunt path across animpedance comprising two elements in series, one of which has arelatively high resistance with respect to said impedance at the normaloperating voltage and a relatively low resistance therebeyond and theother having a relatively high resistance in the direction of currentflow under normal operating conditions and a relatively low resistancein the direction of current fiow opposite to that of normal operatingconditions.

Other features and advantages will be apparent from the followingdescription of the invention, pointed out in particularity by theappended claims and taken in connection with the accompanying drawing,in which the single figure illustrates the application of the inventionto a high power vacuum tube amplifier.

While the invention is applicable to all types of electrical systems inwhich it is desired to prevent overvoltage, it is illustrated here byway of example in connection with a bias voltage circuit for anamplifier stage which may be a portion of a radio transmitter.

The protective control system shown in the figure is associated with theinput circuit of an amplifier system of the push-pull type which findswide application in radio transmitters for the amplification ofmodulation frequency energy. Certain stages of an amplifier of thistype, particularly the final output stage, operates as a class Bamplifier, requiring a compartively large negative bias voltage for thegrid circuit of the tubes. When the output of the amplifier utilizeshigh power tubes, the bias voltage for the grid circuit may reachseveral hundred volts. This voltage is generally derived from a separatepower supply circuit of the alternating current rectifier type includinga filter for eliminating the pulsating form of the rectified current.The filter includes generally an inductance and a capacity, the lattershunting the load resistance of the rectifier, which is usually in thegrid circuit of the amplifying stage. In practice, it was oftenexperienced that the filter condenser mentioned shorted due to suddenhigh voltage caused by a fiashover from the plate of the amplifier tube.While such condensers are designed within safe limits for the articularservice intended, they will not withstand such voltage surges. Theprotective circuit of this invention for preventing the application ofexcessive voltages includes a series network of a gaseous discharge tubeand a rectifier effectively in parallel with the load resistance of thebias supply rectifier.

Referring to the figure, the push-pull amplifier stage comprises vacuumtubes i and 2, each including, respectively, anodes 3 and 3', cathodes 4and 4' and grid electrodes 5 and 5'. The input circuit between grid andcathode of each tube includes the coupling condenser 8 and 8', the inputreactors I and I, and the load resistances 8 and 8'. The reactor 7. andthe load resistor 8 are in series between grid 5 and cathode 4 of tubel. The elements marked with primary indices indicate similar connectionswith respect to tube 2. A bias supply rectifier for the grid circuit oftube l comprises the rectifier tube IS in a full wave rectifying circuitarrangement whereby the anodes II and I! are connected across theterminals of the secondary winding l8 of the power transformer isindicated to be connected to the alternating current supply line. Thecenter tap l6 of the winding l8 connects to the junction point of theresistor 8 and the reactor 1. The resistor 8 is by-passed by condenserl8. The circuit of the rectifier is completed to the cathode I!) throughconductor 28 which is at ground or cathode potential with respect to theamplifier tubes. The bias supply for the tube 2 is similar in all itsdetails to the one just described, the components thereof beingindicated with identical reference characters bearing primary indices.The signal input to the amplifier tubes may comprise any suitable sourceof signal voltage not shown here, which may be connected to theterminals 25, 25' and the common terminal 26. The connection of theseterminals is marked as being to the exclter voltage input.

The output circuit between anode 3 of tube i and anode 3' of tube 2,comprises the center tapped primary winding 28 of the output transformer29. The secondary winding 38 serves as the output source of theamplifier.

The anode voltage source for the amplifier is indicated here by aportion of the rectified power supply comprising the reactor 32 in thepositive side of the voltage source, and filter condensers 33 and 34connected between terminals of the reactor 32 and ground.

The protective circuit comprises two elements, namely, a gaseousdischarge device 48 and a rectifier 4| connected in series across theterminals of condenser l8 and resistor 8. The discharge device 48 has aplurality of electrodes of which 42 and 43 may be connected in paralleland joined by means of conductor 44 with the junction point of thereactor I, the resistor 8 and the condenser 18. Another electrode 45 ofthe discharge device 48 connects to the anode 46 of the rectifier 4|.The cathode 41 thereof is at ground potential being connected also tothe junction point of resistors 8 and 8'. In the other half of thepush-pull input circuit, a second discharge device 48' is connected in amanner similar to the one aforementioned. Electrodes 42 and 43 areconnected by means of conductor 44' to the junction point of reactor 1',resistor 8', and condenser l8' and the third electrode 45' of the device48' is connected by means of conductor 48 to the electrode 45 and alsothe anode 46.

The protective circuit includes, with respect to each filter condenser,current conductive elements which form a series parallel circuit withthe two condensers l8 and I8, namely, the discharge devices 48 and 48and a common current conductive circuit between each series branchcomprising the rectifier 4|. The discharge devices 48 and 48' are of thetype known in the art as protector tubes being non-directional as tocurrent conductivity and exhibit infinitely high impedance below acertain threshold value of potential impressed between any of theelectrodes. Above a critical value, the gas dis- I4, the primary I5 ofwhich 7 asiaooa charge reduces the internal resistance betweenelectrodes to a very low value in comparison with the infinitely highvalue below the critical potential. Gaseous discharge tubes of this typeare described in Patent 1,649,035 to McCoy and are known as, forexample, the Westinghouse type KX642 tubes.

Referring to the operation of the system, normally no current is passingthrough the discharge device 48 or 48' nor the rectifier 4! due to thefact that the voltage across each of the resistors 8 and 8' is normallyof such value which is below the breakdown value of the discharge tubes48 or 48. Furthermore, the rectifier 4| is connected in such polarity asto be non-conductive in the direction of current fiow under normaloperating voltage conditions. The useful function of the rectifier 4i isto prevent current conductivity in the shunt path when the normaloperating voltage across the condenser l8 or I8 is of such value whichwould be sumcient to cause a breakdown between the electrodes of thedischarge device 48 or 48. 'In other words, current conductivity isassured only, in this particular case, when the breakdown is due notonly to excessive voltage but also to a voltage of opposite polarity tothe one which each bias source produces. The polarity of the voltage onthe grid circuit due to a fiashover is positive and is opposite to thenegative bias voltage appearing across the resistors 8 or 8. Wheneversuch a fiashover occurs, the voltage across the bias filter condenser i8or l8 increases and when it reaches the breakdown voltagecharacteristics of the discharge tubes 48 or 48, a breakdown occurs. Thepolarity is now correct for conduction through the rectifier since theanode 46 will be positive with respect to the cathode 41 and therectifier 41 as well as the tubes 48 and 48' now form a conductive pathof low resistance for the current due to excess voltage. In this manner,the fiashover current has a low impedance current path to ground. Whenthe fiashover voltage has been removed from the grid circuit, that is,when normal operation is established, the protective circuit clearsitself and then remains at high resistance until the next fiashover.

I claim as my invention:

1. In an overvoltage protective system, an impedance subjected to aunidirectional potential difierence at predetermined polarity and normaloperating value, a circuit associated with said impedance from whichexcessive voltage magnitudes at reverse polarity are likely to beimpressed thereacross, means for preventing a rise of voltage acrosssaid impedance beyond said normal operating value comprising a currentpath shunting said impedance including in series an element havinginfinite resistance below a threshold value of voltage and relativelylow resistance therebeyond, and another element having infiniteresistance at said operating polarity and relatively low resistance atthe reverse of said polarity.

2. In an overvoltage protective system, an impedance subjected to aunidirectional potential difierence at predetermined polarity and normaloperating value, a circuit associated with said impedance from whichexcessive voltage magnitudes at reverse polarity are likely to beimpressed thereacross, means for preventing a rise of voltage acrosssaid impedance beyond said normal operating value comprising a currentpath shunting saidimpedance including in series an element having arelatively high resistance with respect to said impedance at saidoperating voltage and a resistance much lower than said impedance abovesaid voltage, and a second element having a relatively high resistancewith respect to said impedance at said operating polarity and aresistance much lower than said impedance at the reverse of saidpolarity.

3. In an overvoltage protective system, an impedance subjected to aunidirectional potential difference at predetermined polarity and normaloperating value, a circuit associated with said impedance from whichexcessive voltage magnitudes at reverse polarity are likely to beimpressed thereacross, means for preventing a rise of voltage acrosssaid impedance beyond said normal operating value comprising a currentpath shunting said impedance including in series a gaseous dischargedevice having infinite resistance below a threshold value 01' voltageand relatively low resistance therebeyond and a rectifier connected inconductive relation at the reverse of said polarity having infiniteresistance at said operating polarity and relatively low resistance atthe reverse of said polarity.

4. In an overvoltage protective system, a pair of impedances in serieseach subjected to a unidirectional potential difierence at predeterminedpolarity and opposing with respect to each other at a normal operatingvalue, circuits associated with each of said impedances from whichexcessive voltage magnitudes at reverse polarity with respect to thenormal operating polarity for each of said impedances are likely to beimpressed across said impedances, respectively, means for preventing arise of voltage across said impedances comprising a current pathinterconnecting said series impedances and including as elements a pairof gaseous discharge devices in series, said impedances in said devicesforming a series-parallel circuit and a rectifier forming a currentconductive path common to both said impedances connected betweenjunction points of said elements and said impedances.

5. In an overvoltage protective system, a pair of impedances in serieseach subjected to a unidirectional potential diflerence at predeterminedpolarity and opposing with respect to each other at a normal operatingvalue, circuits associated with each of said impedances from whichexcessive voltage magnitudes at reverse polarity with respect to thenormal operating polarity for each of said impedances arelikely to beimpressed across said impedances, respectively, means for preventing arise of voltage across said impedances comprising a current pathinterconnecting said seriesimpedances and including as elements a pairof gaseous discharge devices in series, said impedances in said devicesforming a series-parallel circuit and a rectifier forming a currentconductive path common to both said impedances connected betweenjunction points of said elements and said impedances, comprising acurrent path shunting each of said impedances including at least onegaseous discharge device for each path and a rectifier common to both ofsaid current paths connected in conductive relation with respect to thepolarity of operating voltage across one of said impedances.

6. In an amplifier system, an amplifier stage of the push-pull typeincluding a pair of electron discharge devices each having anode,cathode and grid electrodes, respectively, a divided output circuitbetween said anodes and cathodes, a divided input circuit between saidgrid and cathodes, said last-mentioned circuit including in seriesbetween grid electrodes and cathodes of each of said devices animpedance and a resistance, respectively, a source of bias voltageconnected across the terminals of each of said resistances, a filtercircuit therebetween including an impedance in shunt with each of saidresistances in the form of a filter condenser, means forv preventingexcessive voltage magnitudes of a reverse polarity to that supplied bysaid source to be impressed across said condensers due to fiashoverbetween said anodes and grid'electrodes comprising a shunt circuit foreach of said condensers including a gaseous discharge device in each ofsaid circuits and a rectifier common to both of such shunt circuits.

JOSEPH E. BAUDINO.

